As employed herein the term "electrochromic display" is intended to include all displays in which the passage of an electrolytic current through one or more display electrodes produces a color or contrast change on or at the electrode(s).
One well known type of electrochromic display achieves such an effect by employing an electrochromic substance in solution which is transparent while dissolved but colored when electrodeposited upon an electrode. The colored and transparent states form a redox pair so that the deposited colored material can be electrolytically removed by reversing the current direction. Perhaps the best known electrochromic substance of this type is the 1,1'-di-heptyl-4,4'-bipyridinium di-cation, which is one of the class of materials known as the viologens. Transparent in solution, it can be reduced electrochemically to the radical cation which is violet colored. In the presence of a suitable anion such as bromide, phosphate or phosphate/hypophosphite mixtures, the colored viologen radical salt precipitates out on the cathode.
The deposition type of display is not the only type of electrochromic display. One other well known type employs a permanent layer of electrochromic material, such as tungsten oxide, on the electrodes. Passage of an appropriate electric charge electrolytically through the electrodes causes a color change in the oxide layer.
In order to control the write and erase operation of both types of displays, it is known to provide in addition to display and counter electrodes, a reference electrode which senses the potential of the solution. Such a reference electrode can be used to control both write and erase operations depending on the particular control scheme selected.
In one known method of controlling a display, selected display electrodes are written to a predetermined contrast by employing a constant current source for a fixed period of time. Under these conditions, a fixed charge is passed and, in a deposition type display, a fixed amount of material is deposited. If the deposit remained absolutely stable upon the display electrodes and conditions remained unchanged, the written display electrodes could be erased by passing an opposite sense constant current for the same period of time. However, many electrochromic deposits slowly redissolve with the consequence that the electrode would be overerased. The forcing of a constant current after all the electrochromic material had been removed would drive the display electrodes more anodic. Depending on the particular materials employed, this could lead to irreversible electrolytic damage to the display electrodes or to damage by liberation of gases within the cell.
Accordingly, the technique of potentiostatic control of erasure has been employed whereby the potential of the counter electrode is controlled with respect to the solution potential as sensed by a reference electrode in the vicinity of the display electrodes. In the method most commonly employed, the reference electrode potential is compared with a predetermined potential corresponding to substantially complete erasure of the display electrodes and the result of the comparison used to control the potential of the counter electrode. Erase current is thus passed through the cell until the reference electrode potential has dropped to the predetermined level. By allowing a small safety margin, over erasure is prevented. The use of reference electrodes in this way is described in a review article entitled "Electrochromic Display" (New Electronics, Sept. 16, 1975, page 66).
A slightly different form of potentiostatic control of erasure is described in our U.S. Pat. No. 4,426,643. In that patent, one of a pair of reference electrodes is always maintained in a written state, i.e. with a deposit of electrochromic material upon it. This stabilizes its potential with respect to the solution. To erase the display, an offset amplifier is employed to drive the display electrodes to a predetermined potential difference from the coated reference electrode, which corresponds to a state of complete erasure.
To complete the review of the prior art, reference is made to U.S. Pat. Nos. 4,255,038, and to 4,306,775 both of which show a type of electrochromic display in which silver in a black, contrasting form, is deposited onto display electrodes from a solution of a silver halide. The U.S. Pat. No. 4,255,038 states the need for erasure of the written silver image to be terminated at the instant when the counter electrode potential approaches a threshold corresponding to a side reaction. How this is detected is not made clear. Only counter and display electrodes are illustrated and the use of reference electrodes is not shown. However, it is stated that the erase voltage may be controlled by means of a reference electrode as described in U.S. Pat. No. 4,306,775. That application shows the passage of a constant current through a reference electrode in order to "measure the impedance" of the cell. Both erasure and writing of the display are said to be servo-controlled with respect to the measured impedance.